Chemical species can be identified by the characteristic features in their infrared absorption and/or transmission spectrum. For light in the mid-infrared (MIR) spectral region of 350-4,000 cm−1, many molecules have characteristic vibrational and rotational energy states that can be populated upon interaction with photons of the appropriate energy (or wavenumber) resulting in absorption and possibly enhanced reflection of light at those specific wavenumbers. This wavenumber specific absorption and enhanced reflection enables the detection of trace amounts of those chemicals by measuring the intensities at various wavenumbers of the light back-scattered from a surface covered by a residue of the chemicals. The back-scattered light can result from the absorption and enhanced reflection processes, which are described by the imaginary part of the residue material's refractive index. The back-scattered light also arises from reflection of light as described by the real part of the material refractive index, including both the light reflected from the various surfaces of the residue and also the light transmitted through the residue and reflected from the underlying substrate surface.
A laser-illuminated active spectrometer can be used to detect and identify chemical residues that may be located on distant surfaces. Chemicals such as highly energetic materials (HEM) have many of their spectral “fingerprint” features within the long-wave infrared (LWIR) spectral range of 800 cm−1 to 1600 cm−1 for which quantum cascade lasers have been demonstrated. These laser sources enable a residue-covered surface to be probed at large stand-off distances because the optical beams formed from the laser outputs can have low-divergence and high power. Also, light from these laser sources can be focused onto small spots, resulting in laser illumination of high brightness and thus higher signal levels for the spectra of the back-scattered light.
Many surfaces, such as the exterior of a vehicle, are highly curved. Thus, a spatially fixed laser source would illuminate those surfaces at a variety of tilt angles, with many of those angles being far from normal incidence (which is perpendicular to the surface). If a detector of the back-scattered light is co-located with or located close to the laser source, the amount of back-scattered light returned to the detector can be very low. For example, the detected back-scattering for relative tilt angles of the surface larger than 5° can be 10−3 to 10−5 that of the signal detected for 0° tilt. Thus, it is beneficial to maximize the power or intensity of the laser illumination to increase the signal. However, the allowable or achievable laser intensity is limited in many cases. In many applications, the laser power must be below the eye-safety limit, which is 0.1 Watts/cm2 for continuous illumination with MIR light. The eye-safety limited laser power can be used more effectively by illuminating the probed surface with only those wavenumbers that are especially relevant for the spectroscopic determination of the chemical species, such as those specific wavenumbers associated with the spectral “fingerprint” features of the chemicals that may or are expected to comprise a residue on the surface.
In the LWIR spectral range, there can be substantial thermal or black-body emission of radiation from many surfaces, including the surface being probed. To reduce the effects of this additional radiation on the spectra detected by the sensor, it is beneficial for the sensor system to collect and couple to its photodetector only the light from the spatial spot on the surface that is being illuminated by the laser source probing that surface. The disclosed framework can control both the location of the laser-probed spot and the location of the spot observed by the photo-detector.
In many cases, the residue on a surface covers only a relatively small portion of the overall surface. Also, there often can be several patches of residue that contain the chemicals of interest, with the areas between the patches not covered with any residue or not covered with those chemicals of interest. A given patch of residue often is spatially continuous and is separated from another patch of residue by residue-free areas of the surface, much like islands in the sea. When the size of the laser-probed spot is smaller than a residue patch, the disclosed sensor system can direct that spot to either be within a residue patch or be in a residue-free portion of the overall surface, which is beneficial for analyzing spectra arising from mixtures of chemical components.
A residue can comprise a mixture of multiple chemicals and the surface itself can contain multiple chemicals. The relative amounts of the chemical species in a mixture typically can be different for different spatial spots of the residue and the thickness of the residue also can vary from spot to spot. Different residue patches can contain different mixtures of chemicals. Each chemical has a unique characteristic spectrum and the materials comprising the surface likewise have their unique spectra. Also, the back-scattering spectra obtained even for a single chemical can vary with the thickness of the residue, the concentration of the chemical, the reflectance of the underlying surface, the roughness of the surface, the roughness of the residue, and the tilt angle. For example, the back-scattering spectrum could resemble the reflectance spectrum of the chemical species in some cases but resemble the transmittance spectrum (or the inverse of the absorbance spectrum) in other cases. The disclosed sensor system framework makes use of this spectral variation and also of the spatial structure of the residue-covered surface to facilitate the selection of the wavenumbers in the illuminating light and also the detection and identification of the chemicals in the residue patches and in the residue-free surface.
U.S. Pat. No. 9,230,302, which is incorporated herein by reference, describes a Foveated Compressive Sensing System for acquiring and reconstructing an optical image. This system makes use of prior knowledge about the image data or about the task to be performed with the imagery to determine the spatial points of the data to be measured and/or retained or the forms used to represent the image data. The resulting image has certain spatial regions that are represented with high information-content, such as high spatial resolution. These regions are called the “regions of interest” (ROI). Other spatial regions of the image are represented with much lower information content. The prior knowledge is used to determine and define the ROI.
In one example, the prior art Foveated Compressive Sensing System operates in a global measurement mode and gathers scene-specific information, such as the intensity of light at observed locations of the scene and the spatial patterns in that intensity distribution, from the entire observed portion of the scene and determines the spatial ROI. This global measurement is done with low spatial resolution. The system is then used in a local “foveated” measurement mode in order to focus the measurement and representation resources on the spatial ROI and on the task-relevant features in those ROI. The “foveated” measurements can provide much higher effective spatial resolution for the portion of the image within the ROI. The system can be switched alternately between global and local measurement modes as required to perform an imaging, recognition or tracking task. Compared to conventional compressive-sensing methods, in which the spatial measurements are made in a random manner, the task-aware sampling of the scene done by this prior art system can reduce the physical number of measurements needed to achieve a given level of task performance.
The present disclosure sensor system is similar to the prior art Foveated Compressive Sensing System in that it likewise makes use of knowledge about the task and the scene to define and determine spatial regions of interest (ROI). However, in the present disclosure it is the spectral resolution that is enhanced within these spatial ROI. Also, the signal-to-noise ratio can be higher for the measured spectra associated with these ROI, since higher illumination power can be applied at each illuminated spectral wavenumber or wavelength when fewer spectral points are illuminated simultaneously.
Most prior art spectrometric sensors make use of ambient illumination or broadband light sources, such as a Globar™ source, and cannot actively control the wavelengths or wavenumbers of the illuminating light. Some prior art spectrometric sensors make use of tunable laser sources for which a physical tuning element, such as a grating, is moved to scan the wavenumber of the emitted light continuously over some spectral span. With such laser sources, it is difficult to hop the wavenumber of the light arbitrarily from one value to another, and that kind of wavenumber or wavelength hopping is not done in practice. The present disclosure framework makes use of laser sources that provide arbitrary hopping of the laser emission from one selected value to the next, and that can emit multiple selected wavelengths simultaneously.
What is needed is an improved spectral sensor system and method. The embodiments of the present disclosure answer these and other needs.